Cellular wireless communications systems such as those defined by the General Packet Radio Service (GPRS) and its successors provide communications for mobile systems (MS) (e.g., phones, computers, or other portable devices) over a service provider's core network or backbone by means of base stations (BS) connected to the core network that relay communications to and from the MS via a wireless link. The geographic area over which a particular BS is able to communicate wirelessly (i.e., via the air interface) is made up of one or more zones of radio coverage referred to as cells. In order to perform a data transfer, an MS connects to the network in a serving cell hosted by a specific BS of which it is in range. The connection may move to other cells served by the same or by a different BS when conditions warrant (e.g., a change of location as the MS moves from one cell to another) with a handover process.
A BS may provide uplink channels and/or downlink channels for multiple MS's by time division and frequency division multiplexing. In a GPRS system, for example, the BS may periodically broadcast bursts of control data on a defined broadcast control channel (BCCH) over one or more frequency channels that divide time into discrete segments called frames and contain time slots used for data transmission between the BS and an MS. The time slots of each frame on each defined frequency channel constitute the physical channels through which data transfer between an MS and a BS takes place. Logical channels, defined by the type of information they carry, may be further defined as corresponding to particular physical channels and are used to carry traffic (i.e., voice or packet data) and control data in uplink and downlink directions. The MS listens to control signals broadcast by the BS and maintains synchronization therewith in order to receive and transmit data over particular logical channels.
In order for an MS to initiate access to the network, either to initiate a data transfer or respond to a page from the BS sent over a paging channel, it may contend for medium access by transmitting an access request message to the BS on a particular channel defined for that purpose, referred to in GPRS as a random access channel (RACH). If the access request message is successfully received, the BS responds over an access grant channel and assigns downlink and/or uplink channels that are used to transfer data between the BS and MS. The assigned downlink or uplink channels constitute a virtual connection between the BS and MS that lasts for the duration of the data transfer in the cell the MS is camped, referred to as a temporary block flow (TBF).
Although the MS can maintain synchronization with downlink frames received by it from the BS, synchronization of uplink frames transmitted by the MS with the BS requires that the propagation delay be taken into account. The MS may therefore transmit its data with a specified timing offset that corresponds to the time it takes for a signal to reach the BS, referred to as the timing advance (TA). The TA is essentially a negative offset, at the MS, between the start of a received downlink frame and a transmitted uplink frame. The BS can determine the appropriate TA from the arrival times of signals transmitted by the MS with a predetermined TA (e.g., a TA value of 0 corresponding to no timing advance or some other predetermined TA value) and communicate this information to the MS.